As the dimensions of semiconductor devices continue to shrink, accurate and efficient characterization of the components forming those devices becomes more critical. Typically, the manufacturing process for modern semiconductor devices includes the formation of a number of layers or “thin films” on a silicon wafer. The thin films can include oxide, nitride, and/or metal layers, among others. To ensure proper performance of the finished semiconductor devices, the thickness and composition of each thin film formed during the manufacturing process must be tightly controlled.
Single wavelength ellipsometry (SWE) is a technique for measuring thin film thickness by directing a single wavelength polarized beam at a thin film, and then measuring the change in polarization state of the reflected beam, as described in co-owned, co-pending U.S. patent application Ser. No. 09/298,007 (“System for Analyzing Surface Characteristics with Self-Calibrating Capability”, Wang et al.), herein incorporated by reference. SWE is the most stable and reproducible measurement (metrology) technique for thin film thickness, and is therefore the technique of choice for measuring critical structures such as gate oxides.
Unfortunately, certain modern process techniques can create problems for conventional SWE. For example, FIG. 1 shows a basic silicon-on-insulator (SOI) wafer 100, which can be used to improve the speed of metal-oxide-semiconductor (MOS) transistors in an IC. SOI wafer 100 includes a gate oxide layer 120 formed on an SOI substrate 110. SOI is formed by a silicon wafer 111, a buried insulator layer 112 formed on silicon wafer 111, and a superficial silicon layer 113 formed on buried insulator layer 112. Buried insulator layer 112 isolates devices formed in superficial silicon layer 113 from silicon wafer 111, thereby eliminating junction capacitance and allowing those devices to operate at a higher speed.
Modern SOI processes typically use a buried insulator thickness of somewhere between 500 Å and 2000 Å, and a superficial silicon layer thickness that is less than 500 Å. Device speed can be further improved by scaling the dimensions of the superficial silicon layer, and some advanced processes reduce the superficial silicon layer thickness down to roughly 200 Å. As the SOI dimensions are reduced, the thin film layers formed on SOI substrate 110 must also be scaled so that the full performance benefits can be reaped.
As SOI substrate dimensions decrease, the accuracy with which thin film layers (such as gate oxide layer 120) are formed on the SOI substrate becomes ever more critical. However, the lasers used by conventional SWE systems to generate measurement beams are ill-suited for SOI constructions. The measurement beam in a SWE system is partially reflected and partially transmitted by a thin film. The transmitted portion of the measurement beam is then partially reflected and partially transmitted by the substrate beneath the thin film. The beams reflected by the thin film and the substrate constructively and destructively interfere, thereby producing the characteristic output (reflected) beam used to determine the thickness of the thin film.
For thin films produced on conventional silicon substrates, the portion of the measurement beam transmitted by the substrate is eventually absorbed by the silicon substrate, and therefore does not affect the SWE measurement. However, any portion of the measurement beam transmitted by an SOT substrate can be reflected at the various layer interfaces of the multi-layer SOT substrate. Such spurious reflections can then alter the characteristic output beam, thereby degrading the accuracy of the SWE measurement.
Conventional SWE systems use lasers that generate beams in the visible or IR range, since such lasers are readily available and work well for gate oxide measurements (on monolithic silicon substrates). Often, the laser in a conventional SWE system is a helium-neon (He—Ne) laser operating at 632.8 nm or a yttrium-aluminum-garnet (YAG) lasers operating at 532 nm (frequency doubled). However, the absorption distance in silicon (i.e., the depth to which the beam will penetrate a material before the intensity of the beam is reduced to 1/e of its original level, where “e” is the natural logarithmic base, roughly equal to 2.718) for beams at these wavelengths is in the 600–1000 Å range. Such measurement beams are unacceptable for thin film measurement on SOI substrates, since the measurement beams will fully penetrate the 200–500 Å superficial silicon layer and therefore generate the spurious reflections that lead to measurement inaccuracy.
Consequently, spectroscopic ellipsometry (SE), rather than SWE, is used to measure thin films on SOT substrates. SE, such as described in co-owned U.S. Pat. No. 5,608,526 (“Focused Beam Spectroscopic Ellipsometry Method and System,” issued Mar. 4, 1997 to Piwonka-Corle et al.), involves scanning a wide range of wavelengths simultaneously. The reflected radiation therefore includes multiple frequency components, allowing a spectrum of measured data to be read. From this spectrum the thicknesses of multiple layers in a material stack can be determined. To measure a thin film on an SOI substrate, SE is performed and the data associated with the SOI layers is discarded, leaving only the thin film information.
While this “selective” use of SE data allows thin films on SOT to be measured, SE is not the ideal technique for measuring single thin films (such as a gate oxide layer). The multiple measurement beam frequencies used in SE increase processing time (and therefore measurement throughput), and also reduce the accuracy of the actual measurement, since the effects of interactions between the multiple wavelength beams cannot be eliminated completely. Also, the complexity of SE hampers system-to-system matching because it is difficult to precisely restrict the range of wavelengths used in a SE system.
Another problem that impedes accurate thin film measurement on SOT substrates is contaminant layer growth. Modern thin films have reached the point where the accuracy and reproducibility of thin film measurements can be limited by contamination on the surface of the thin film. For example, airborne molecular contamination (AMC) such as water and other vapors can be absorbed onto the thin film, creating a contaminant layer that adversely affects optical ellipsometry (both SE and SWE).
Conventional methods for cleaning thin films include heating the entire wafer in an oven to a temperature of about 300° C. to vaporize any contaminant layer, as described in U.S. Pat. No. 6,325,078 (“Apparatus and Method for Rapid Photo-Thermal Surface Treatment,” issued Dec. 4, 2001 to Kamieniecki), and placing the wafer on a heated stage, as described in U.S. Pat. No. 6,261,853 (“Method and Apparatus for Preparing Semiconductor Wafers for Measurement,” issued Jul. 17, 2001 to Howell et. al.). However, these bulk heating systems require large thermal control components (e.g., lamps, heated stages, heat exchangers, etc.) that undesirably increase the cleanroom space required for these conventional cleaning systems. Furthermore, the long heatup and cooldown times required by bulk heating systems can significantly reduce throughput (as does the time required for transferring the wafer to and from the cleaning system). In addition, contaminants can redeposit on the cleaned wafer while it is being transferred from the cleaning system to the film analysis tool.
To improve throughput and reduce system footprint, a laser cleaning system can be incorporated into a metrology system, such as described in co-owned and co-pending U.S. Provisional Patent Application Ser. No. 60/426,138 (“Film Measurement with Interleaved Laser Cleaning,” filed Nov. 13, 2002 by Janik), herein incorporated by reference. By including both a measurement beam source and a cleaning beam laser in a single metrology system, localized cleaning and measurement can be performed simultaneously or in rapid sequence to improve throughput. Furthermore, since a separate heating chamber is not required, total system footprint can be reduced. However, the use of multiple beam sources requires multiple alignment mechanisms, which increases system complexity.
Accordingly, it is desirable to provide a method and system for accurately measuring thin films on an SOI substrate that avoids the aforementioned problems associated with AMC contamination and regrowth, while minimizing system complexity.